Origins
The word 'monument' entered English in the 13th century from Old French 'monument,' which descended from Latin 'monumentum' (a memorial, a reminder, a record, a tomb). The Latin form is a contraction of earlier 'monimentum,' built from 'monēre' (to warn, to advise, to remind) with the instrumental suffix '-mentum' (the means or result of an action). A 'monumentum' is therefore, literally, 'that by which one is reminded' — a device for memory.
The Latin verb 'monēre' derives from PIE *men- (to think, to remember), one of the most productive roots in Indo-European. The same root produced Latin 'mēns' (mind), 'memor' (mindful), 'memoria' (memory), and 'monitor' (one who warns), as well as Greek 'mnēmē' (memory) and 'mnēmeîon' (memorial) — which is itself the etymological twin of 'monument,' both meaning 'an instrument of remembering' but arriving from different branches of the same root.
In Classical Latin, 'monumentum' had a broader range than its English descendant. It could mean a written record or literary work (Cicero spoke of the 'monumenta' of the Republic's laws), a tomb or burial place, or a physical memorial. The 'tomb' sense was especially strong in Medieval Latin and survives in modern French, where 'monument funéraire' means a funerary monument or gravestone. In English, the 'tomb' sense was active through the 17th century — Shakespeare uses 'monument' to mean 'tomb' in Romeo and Juliet ('Shall I believe / That unsubstantial Death is amorous, / And that the lean abhorred monster keeps / Thee here in dark to be his paramour?').
Modern Usage
The modern English sense — a structure deliberately erected to commemorate a person, event, or achievement — solidified during the 17th and 18th centuries, an era of monumental architecture across Europe. The Monument in London (built 1671–1677) commemorating the Great Fire of 1666 is simply called 'The Monument,' so strongly had the word become associated with commemorative structures.
The adjective 'monumental' (from Latin 'monumentālis') extended the word figuratively: something 'monumental' is not just large but significant, lasting, worthy of being remembered — 'a monumental achievement,' 'a monumental error.' This figurative sense, attested from the early 17th century, preserves the original Latin emphasis on memory and lasting importance.
German took a different path with this concept: 'Denkmal' (think-sign, from 'denken,' to think, + 'Mal,' mark or sign) is a calque that independently captures the same idea — a mark that makes you think. Modern Greek 'μνημείο' (mnimeío) descends from the Greek cognate of the same PIE root. Across languages, the concept of a monument is consistently anchored to the idea of enforced remembrance — a physical object whose purpose is to make the mind do its work.